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std::nullptr_t(3)	       C++ Standard Libary	       std::nullptr_t(3)

NAME
     std::nullptr_t - std::nullptr_t

Synopsis
	Defined in header <cstddef>
	using nullptr_t = decltype(nullptr);  (since C++11)

	std::nullptr_t is the type of the null pointer literal, nullptr. It is a
     distinct
	type  that is not itself a pointer type or a pointer to member type. Its
     values are
	null pointer constants (see NULL), and may be  implicitly  converted  to
     any pointer
	and pointer to member type.

	sizeof(std::nullptr_t) is equal to sizeof(void *).

Notes
	The  C++ standard requires <stddef.h> to place the contents of <cstddef>
     in the
	global namespace, and thereby requires nullptr_t to be available in  the
     global
	namespace when <stddef.h> is included.

	nullptr_t is not a part of C until C23.

	It is unspecified whether the declaration of std::nullptr_t is available
     in any
	other  standard  library header. An implementation may avoid introducing
     this name
	even when the standard requires  std::nullptr_t  to  be  used,	by  e.g.
     spelling
	decltype(nullptr) instead.

Example
	If two or more overloads accept different pointer types, an overload for
	std::nullptr_t is necessary to accept a null pointer argument.

     // Run this code

      #include <cstddef>
      #include <iostream>

      void f(int*)
      {
	 std::cout << "Pointer to integer overload\n";
      }

      void f(double*)
      {
	 std::cout << "Pointer to double overload\n";
      }

      void f(std::nullptr_t)
      {
	 std::cout << "null pointer overload\n";
      }

      int main()
      {
	  int* pi{};
	  double* pd{};

	  f(pi);
	  f(pd);
	  f(nullptr); // would be ambiguous without void f(nullptr_t)
	  // f(0);    // ambiguous call: all three functions are candidates
	  // f(NULL); // ambiguous if NULL is an integral null pointer constant
		      // (as is the case in most implementations)
      }

Output:
      Pointer to integer overload
      Pointer to double overload
      null pointer overload

See also
	nullptr(C++11)	the pointer literal which specifies a null pointer value
	NULL		implementation-defined null pointer constant
			(macro constant)
	is_null_pointer checks if a type is std::nullptr_t
	(C++14) 	(class template)
	C documentation for
	nullptr_t

http://cppreference.com 	   2024.06.10		       std::nullptr_t(3)
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